A technique for rapid detection of spread spectrum sequences
C. S. Miller · 1997
In systems using direct sequence spread spectrum signals (DSSS) it is often necessary to monitor activity and to detect the presence of code sequences of interest. This might occur in the case of a relay where it is necessary to recognize and sort DSSS signals so that they can be processed and/or routed. Instead of the straightforward detection approach of correlating received signals against a set of a priori known references we use a feedforward shift register approach where the shift register taps are chosen to implement the inverse polynomial of the DSSS code sequence. The output of the inverse polynomial, ideally, is a constant which, after integration, can be threshold tested to determine signal presence. The advantage of this approach is that it avoids searching the time phase of the spreading sequence saving, perhaps, as much as a factor of one thousand over the conventional method. Examples of the application of the technique include the detection of a length 1023 pseudo-noise (PN) sequence and the detection of 127 bit Gold codes. The technique allows the detection of the presence of any of the 129 Gold codes with a single pass. The technique is not intended for low probability of intercept applications where the signal is buried deep in the noise. A signal-to-noise (S/N) ratio or signal-to-interference ratio with error rate less than about 20% is required. For coherent binary phaseshift keying this corresponds to about -5 dB. Examples are presented with plots of the performance.